I don’t know about its top speed, but I’m pretty sure it can break the sound barrier.
In Tale Of The Toa, Pohatu is able to run from Po-Koro to the base of Mount Ihu extremely quickly; reading the book makes it seem like just a few seconds, while BS01 only says “under a minute”.
We know from Makuta’s Guide To The Universe that Mata Nui is 486 km from tip to tip, and the distance from Po-Koro to Mount Ihu is approximately a sixth of that distance; let’s lowball it and say he only ran 75 km.
Even with the most conservative estimate of 75 km in 60 seconds, Pohatu ran at an average speed of 4500 km/h, which is more than 3 times faster than the speed of sound (1234.8 km/h).